Field of the Invention
The present invention relates to an optical part and a method of manufacturing the optical part. More particularly, the present invention relates to, for example, an optical part that is a lens with a holder, used in a finder of a digital single lens reflex camera; and a method of manufacturing the optical part.
Description of the Related Art
Resin lenses have many uses, such as being used as a photographing lens of a digital camera, a recording/reproducing pickup lens of an optical disk, and a projection lens of a projector. To reduce costs, many of these resin lenses are manufactured by injection molding. To install such resin lenses as parts in a product and make them function, such lenses are placed in a frame-shaped lens holder and integrated therewith in a back end process. Although the lens holder is also called a lens barrel, the lens holder is consistently called a holder in the present invention. The holder functions as a part that allows a resin lens to be easily installed in a camera, functions as a driving part that moves the resin lens, and functions to intercept light (functions as a mask) to suppress the occurrence of ghosts caused by reflection of light by a side surface of the lens.
To fix the lens to and integrate the lens with the holder, an adhesive is generally used. However, the cost of the back end process of fixing the lens to and integrating the lens with the holder with an adhesive is high. A method of molding a lens by, with an undercut being formed in an inner periphery of the holder and the holder being inserted in a mold, pouring molten resin therein is also used. With the undercut being formed in the holder, the molten resin flows around the undercut. When the molten resin is solidified in this state, while the lens is firmly fixed to and integrated with the holder, an optical part that is the lens with the holder can be taken out from the mold.
According to Japanese Patent Laid-Open No. 2002-148501 (Patent Literature (PTL) 1), a light-intercepting section (a mask section) is formed in addition to forming an undercut section for fixing a lens in an inner periphery of a resin holder. PTL 1 describes the molding of a resin lens with the resin holder being inserted in a mold.
FIG. 8 is a figure for describing PTL 1 and is a sectional view of the lens with the holder described in PTL 1. The sectional view is a view formed by cutting the lens with the holder by a parallel plane that includes an optical axis of the lens and that is parallel to the optical axis. In FIG. 8, the entire region of a side surface 811 of a resin lens 81 is surrounded by a resin holder 82. An undercut section 85 and a light-intercepting section (a mask section) 812 exist in the inner periphery of the resin holder 82. When a lens material flows around the undercut section 85 and the light-intercepting section 812 and is solidified, the undercut section 85 and the light-intercepting section (the mask section) 812 penetrate the side surface 811 of the lens. By causing the holder 82 to penetrate the lens in the direction of the optical axis of the lens, the holder is fixed to the lens.
When reference numeral 83 denotes an eyepiece-side lens surface and reference numeral 84 denotes an objective-side lens surface, a reflection light beam 88 that is incident from the objective-side lens surface 84 and that is reflected at the lens side surface 811 is intercepted by the light-intercepting section (the mask section) 812, and, thus, does not reach the eyepiece-side lens surface 83. Similarly, a reflection light beam 89 that is incident from the objective-side lens surface 84 and that is reflected at a side surface 86 of the undercut section 85 is also intercepted by the light-intercepting section (the mask section) 812 and, thus, does not reach the eyepiece-side lens surface 83.
However, a reflection light beam 810 that is incident from the eyepiece-side lens surface 83, that is visually recognized when seen from the eyepiece-side lens surface 83, and that is reflected at a front surface 87 of the undercut section 85 is transmitted through the eyepiece-side lens surface 83 and travels back to an eye side. Therefore, the reflection light beam 810 is visually recognized as a ghost. Similarly, since the resin of the lens flows around the entire mask 812, a reflection light beam 817 that is incident from the eyepiece-side lens surface, that is visually recognized when seen from the eyepiece side, and that is reflected at a planar section 818 of the mask 812 is generated. The reflection light beam 817 is also visually recognized as a ghost because the reflection light beam 817 is also transmitted through the eyepiece-side lens surface 83 and travels back to the eye side.
When a lens is molded out of molten resin with the holder inserted in the mold, the eyepiece-side lens surface 83 and the objective-side lens surface 84 are cooled while they are in contact with a mold surface, whereas the lens side surface 811 is cooled while in contact with the holder 82. Since the holder 82 is made of resin, the holder 82 has a thermal conductivity that is lower than that of metal. This hampers heat exchange from the lens side surface in the cooling process. Since the heat exchange of the entire lens is not uniform, a mold mirror surface cannot be transferred with high precision.
Further, since it is necessary to insert and hold the holder 82 in the mold, the holder 82 needs to be larger than the external shape of the lens. Therefore, the optical part becomes large.
As described above, hitherto, in manufacturing a lens with a holder, there has been a demand for reducing the size of an optical part, reducing the occurrence of ghosts, and increasing the precision of lens surfaces.